1. Field of the Invention
The present invention relates to a pressing mechanism for a transfer device in an image forming apparatus.
2. Description of the Related Art
A technology of using a pressing mechanism when transferring an image from an image carrier onto a recording medium is widely known in the field of an image forming apparatus such as a copier, a printer, a facsimile, and a multifunction peripheral thereof, such as one disclosed in Japanese Patent Application Laid-open No. 2007-148196. The pressing mechanism presses a contact unit such as a secondary transfer roller that is brought into contact with a contact target unit such as an intermediate transfer belt or a secondary transfer opposing roller.
In the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2007-148196, four photosensitive elements are aligned to oppose an intermediate transfer belt. Toner images of four different colors of black, yellow, magenta, and cyan are formed on the photosensitive elements, which are transferred onto the intermediate transfer belt in a superimposed manner. Then, the toner images formed on the intermediate transfer belt are transferred as a color image onto a recording medium such as a print sheet at a position where the intermediate transfer belt and the secondary transfer roller are brought into contact with each other.
In such image forming apparatus, a contact/separation mechanism is provided for bringing the secondary transfer roller into contact with the intermediate transfer belt and separating the secondary transfer roller from the intermediate transfer belt. The contact/separation mechanism enables a recording medium (sheet) jammed at a nip portion between the intermediate transfer belt and the secondary transfer roller to be removed easily and facilitates assembly and maintenance when the intermediate transfer belt and the secondary transfer roller are each configured to be detachable as a unit from the image forming apparatus. When the secondary transfer roller is in contact with the intermediate transfer belt (i.e., in normal times), the pressing mechanism presses the secondary transfer roller against the secondary transfer opposing roller with the intermediate transfer belt therebetween.
More specifically, in Japanese Patent Application Laid-open No. 2007-148196, the secondary transfer roller is held by a holding unit (auxiliary rotating unit). The holding unit is supported at three points, by the engagement of a reference pin with an insertion hole formed in an end portion of the holding unit in its width direction and the engagement by two coil springs provided on both end portions of the holding unit in the width direction. Under this condition, the secondary transfer roller is pressed against the intermediate transfer belt.
In the above technology, the secondary transfer roller is pressed against the intermediate transfer belt while being held by the three-point supported holding unit. Therefore, the secondary transfer roller is expected to be pressed against the intermediate transfer belt (or the secondary transfer opposing roller) in a relatively uniform manner in the width direction. However, even with such configuration, the pressure on the secondary transfer roller against the intermediate transfer belt does not always become even and varies in the width direction in some cases, which leads to unevenness in a toner image transferred onto a recording medium by the secondary transfer roller. This results in unevenness in density of an output image in the width direction, i.e., a direction perpendicular to a recording medium conveying direction.
The above problem is noticeable especially in an image forming apparatus capable of feeding a recording medium of a large width.
Moreover, the above problem is not limited to a pressing mechanism in which a secondary transfer roller is used as a contact unit and a secondary transfer opposing roller or an intermediate transfer belt is used as a contact target unit, and occurs to any pressing mechanism that presses a contact unit against a contact target unit.